It is a well-known phenomenon that many metals, alloys and chemical compounds substantially lose all of their electrical resistance and become superconductive at temperatures near absolute zero. This phenomenon is advantageously employed in electrical alternators or generators by cooling the rotor of the generator to a sufficiently low temperature. By operating a generator in its superconducting state, losses in the windings of the rotor are substantially eliminated and the generator efficiency is correspondingly increased. Additionally, the generator rotors and stators can be built to a much smaller dimension. This results in a reduction in weight of the generator. Moreover, when the rotor is smaller, there is a resultant reduction in operating problems such as vibrations, high material stresses and the like frequently encountered in high-speed rotors.
In general, superconducting rotors consist of a number of cylindrical concentric elements. On the outside there is a damper shield and a damper shield support which are supported by an outer rotor. Inside the outer rotor is an inner rotor including superconducting field windings or coils immersed in a helium refrigerated annulus. This helium refrigerated annulus typically maintains the temperature of the superconducting coils at 4.3.degree. K. or below so that superconductivity takes place. Intermediate the two rotors and concentric therewith, is a thermal radiation or insulating shield, designed to minimize radiant heating of the cold inner rotor. The term "rotor" will be used to refer to the inner rotor, the outer rotor, and the insulating shield.
An inherent problem in the design of superconducting generators is the accommodation of the relatively large thermal deflections between the cold inner rotor and the relatively warm outer rotor and between the cold inner rotor and the insulating shield. The present invention accommodates these differential axial thermal expansions and contractions.
The outer damper shield and damper shield support serve two functions. First, they comprise the strongback outer thermal jacket of the super-cooled rotor. Second, they prevent back electromotive forces from the stator from penetrating to the superconducting coils. If penetration to the superconducting coils of the back electromotive forces occurs, the coils of the windings become heated. When they become heated above a critical temperature, they lose their superconductivity and the designed field is lost.
During normal operation, the inner rotor is first subjected to "cool-down". In cool-down, liquified helium is introduced into the vicinity of the superconducting coils. The inner rotor undergoes substantial thermal contraction in an axial direction. Taking the case of a superconducting rotor 132 inches long, a thermal contraction of 3/10 of an inch or more can occur. In a longer superconducting rotor on the order of 275 inches long, thermal contractions of as much as 7/10 of an inch or more can occur. Simultaneously, the insulating shield, which will be cooled to an intermediate temperature of about 100.degree. K., contracts axially, but normally in an amount less than that of the inner rotor.
At the same time this axial shrinkage is accommodated, any tendency of the inner rotor to move rotationally with respect to the outer rotor must be prevented. Otherwise, this relative movement between inner and outer rotors will generate undesired back electromotive heating of the superconducting coils and can result in the loss of their superconductivity.
Further, any tendency of the rotors to move out of concentric alignment must be avoided. Even a minute eccentricity of the rotors may result in substantial resonances and unbalanced forces during high-speed rotation. Therefore, the connection must have sufficient lateral (radial) stiffness and strength to maintain the rotors in concentric alignment.
In addition, the regions between the rotor elements are in a vacuum which adversely affects the operation of a sliding coupling. At low vacuum temperatures and at high rates of rotation, a rapid "fretting corrosion" of the sliding parts normally occurs. Also, in a vacuum, rubbing surfaces frequently gall and seize or weld.
In the past, it has been proposed to connect the rotor elements rigidly both axially and torsionally. However, this design leads to excessive axial stresses in large generators.
Prior art patents do not address themselves to the particular needs of a coupling between the rotors of a superconducting generator. There are, however, a number of prior art patents which disclose a variety of couplings designed to connect misaligned shafts in end-to-end relation. Typical of these are U.S. Pat. Nos. 3,798,924, 3,874,195, 3,759,064, 3,703,817, 1,947,052, and 3,405,760. None of the couplings disclosed in these patents is adapted to solve the problems encountered when connecting concentric structures of a superconducting rotor.
In general, the couplings disclosed in these patents provide a driving connection only between the shafts since the shafts are supported by independent bearings on each side of the coupling. These couplings are well adapted to accommodate misalignments between rotating shafts. However, they are generally incapable of accommodating appreciable axial movement between the shafts, particularly at the low temperatures encountered in super-cooled generators and they are even less capable of radially supporting a pair of shafts, especially when the weight of the rotary element is as large as that of the rotor of a generator.